1. Field of the Invention
The present invention relates to a glass substrate that is formed by cutting glass with laser light radiation and forced cooling, and to a glass cutting method.
2. Description of Related Art
In general, as a glass cutting method for cutting a glass plate vertically and horizontally to produce a glass substrate with predetermined width and length, there is a mechanical cutting method as described in, for example, Japanese Patent Publication No. 11-71124, or a laser cutting method for using laser light radiation is employed.
In the case where a cutting method with a mechanical process is employed, a blade pressurizes glass to generate median cracks, that is, cracks propagating vertically onto a glass surface, thereby cutting the glass. In this cutting method, the thus formed glass cut faces are brought into contact with each other in a depressurization step of the blade, resulting-in generation of lateral cracks, that is, cracks propagating horizontally on a glass surface.
Therefore, it was conventionally difficult to perfectly prevent the generation of microcracks, that is, lateral cracks generated on a cut face in a mechanical glass cutting process. The generation of cracks also leads to generation of a pulverized powder, so-called cullet. As a result, inconveniences occur in that, for example, the cullet covers the surface of a display and impairs the visibility. Moreover, in the case where a glass substrate is used for a touch panel or the like, the cullet adheres to a transparent conductive layer and causes short circuit.
Although there is a method for reducing the generation of such lateral cracks through a chemical treatment, such a method wanes the productivity, leading to increased cost. Moreover, the use of a chemical treatment has a problem in that it generates a variation in glass strength.
On the other hand, as a laser cutting method, for example, a heat cutting method with a carbon dioxide gas laser has been proposed in Japanese Patent Publication No. 8-217478, Japanese Patent Publication No. 11-254172 and the like.
In the case where a cutting method only with normal laser light radiation is used, however, on a cut side face of the glass substrate 1 which is formed by radiating laser light, minute cracks and cullet are adversely generated in a laser mark 2 that appears from the laser-irradiated side, as represented by a schematic sectional structure showing an example thereof in FIG. 8.
To cope with such a problem, a cutting method, which takes advantage of combined effects of heating through radiation of a carbon dioxide gas laser or the like and forced cooling using water or a compressed air, has been proposed. According to this method, a thermal strain is generated in glass owing to the combined effects and is used as a trigger to cut a glass to be separated.
In particular, this cutting method incorporating laser radiation and forced cooling attracts attention as a cutting method for obtaining a thin glass substrate having a thickness of 1.1 mm or less, for example, about 0.3 to 0.7 mm. Such a thin glass substrate is utilized for a touch panel that is recently attached on a surface of various displays as a simple way of inputting information or for a portable terminal using such a touch panel.
This method is advantageous in that microcracks are scarcely generated because no mechanical force is applied to a glass.